1. Field of the Invention
The present invention relates generally to ballasts for compact fluorescent lamps, and relates more particularly an adaptive ballast control for a compact fluorescent lamp in an integrated circuit.
2. Description of Related Art
Electronic ballasts for fluorescent lighting applications are widely available and include integrated circuits that perform standard functions, application specific integrated circuits that provide additional functionality and micro controllers that permit programmable control of fluorescent lamps. The functions performed by these controllers often include power factor correction and lamp and/or ballast control. These types of controllers can be provided in small packages and often contribute to meeting technical requirements in newly developed fluorescent lamps while reducing the number of components in the electronic ballast, and consequently achieve a reduction in costs. A lamp type that has recently gained in popularity is a compact fluorescent lamp or CFL, which typically includes a ballast with the lamp structure, rather than separately provided from the lamp. In a typical CFL electronic ballast, the circuit with self oscillating bipolar transistors is provided to achieve the design goals of low cost, low component count and smaller package size. The self oscillating bipolar transistor design is often preferred for a number of applications involving CFLs.
However, the self oscillating bipolar transistor solution suffers from a number of drawbacks that can complicate the use of the design with a number of desired applications. For example, the bipolar design is not self starting, but rather requires a diac and additional circuitry to start the lamp. Rather than obtaining an equivalent free wheeling diode in a body of a MOSgated transistor, for example, the bipolar transistors use additional free wheeling diodes connected across an emitter and a common terminal, adding to the part count and cost. The operating frequency of the bipolar design is determined by the bipolar transistor charge storage time and the saturation of a toroid typically used with the ballast. In addition, preheat used for starting a CFL is provided by a thermistor that is somewhat unreliable and “always hot” with a positive temperature coefficient (PTC) in the bipolar design. The bipolar design also does not provide a feature for smoothly ramping up a circuit frequency during ignition, which would otherwise be useful to prevent wear on components that can occur during ignition with the bipolar solution.
Typically, the bipolar solution does not permit fault detection and responsiveness, especially in the case of a non-starting lamp, or an open filament in the lamp. The bipolar solution also has a drawback that it operates in a capacitive mode, which does not achieve the highest efficiency available. Furthermore, the bipolar solution is limited to lower power applications, due to base drive limitations that prevents its use with higher power applications.
The above mentioned drawbacks, while not presenting a major issue for a given application in and of themselves, tend to produce design difficulties in practice. When taken together, these drawbacks can produce failures and problematic operation, including a high susceptibility to component and load tolerances. Problems with the lamp or ballast components can result in catastrophic failure of the ballast output stage components, especially with regard to lack of tolerance for faults or circuit operation that is out of conformance with specifications. Because the bipolar solution lacks resilience with regard to fault handling, and has a higher component count, performance in practice can be poor for certain applications, resulting in a poor quality system or field failures. Accordingly, there is a need for a CFL electronic ballast that overcomes the drawbacks of the prior art.